The present invention relates to a powder-dispensing device.
Accordingly, embodiments of the present invention relate in particular to a powder-dispensing device in the form of a dense phase powder pump or a powder injector for conveying coating powder from a powder reservoir to a powder-spraying device. Embodiments of the invention relate in particular to both those powder-dispensing devices and powder spray coating devices comprising an injector as the powder pump as well as those comprising a dense phase powder pump.
Dense phase powder pumps in the sense of the present disclosure have at least one feed chamber provided with a powder intake valve and a powder outlet valve. The feed chamber can be alternatingly connected to a vacuum source during a suction stage or to a source of compressed conveying air during a dispensing stage. The vacuum of the vacuum source sucks powder in the feed chamber through the open powder intake valve while the powder outlet valve is closed. The compressed conveying air from the compressed conveying air source dispenses the powder from the feed chamber through the open powder outlet valve while the powder intake valve is closed.
According to one aspect of the present invention, the powder-dispensing device is designed as a single-chamber dense phase powder pump, whereby the dense phase powder pump only comprises one single powder feed chamber for conveying the coating powder.
According to a further aspect of the invention, however, a dense phase powder pump is provided which makes use of at least two feed chambers connected in parallel. With the use of at least two parallel-connected feed chambers, powder is in each case sucked into one of the at least two feed chambers while powder is dispensed out of the other of the at least two feed chambers, whereby the powder is then dispensed from the one feed chamber and new powder is sucked into the other chamber. The parallel connection of two or more feed chambers can produce a continuous flow of powder in a powder-dispensing line connected to the powder outlet valves of all the feed chambers, particularly without supplementary compressed conveying air needing to be fed into the powder path in phase opposition to the powder dispensing after the feed chamber.
Different types of powder-dispensing devices implemented as dense phase powder pumps are known, for example from the following documents: JP 09/071325 A, DE 196 11 533 B4, EP 1 644 131 A2, WO 2004/087331 A1 and EP 1 566 352 A2.
These dense phase powder pumps known from the prior art have at least one, usually two feed chambers. A vacuum inlet of the feed chamber, in some embodiments also a compressed air inlet of the feed chamber, is provided with a filter which is permeable to air but impervious to coating powder. It is customary to use sintered material as the material for the filter. The powder intake valve and the powder outlet valve are usually pinch valves, which have already proven effective in combination with injector pumps in dilute phase conveying of powder because they are less prone to powder depositing in them and because the gas flowing through them, normally a flow of pressurized air, makes them easier to clear than other types of valves.
The amount of powder delivered by a dense phase powder pump per unit of time is particularly dependent on the size (volume) of the feed chamber, the frequency at which coating powder is sucked into the feed chamber and then dispensed from it again, the force of the vacuum for the length of time the powder intake valve is open during the suction phase, and the flow resistances in the powder lines upstream and in particular downstream of the dense phase powder pump. The flow resistances are particularly dependent on the length and the internal cross section of the powder supply lines, usually powder hoses. The compressed conveying air only mixes minimally with the coating powder and pushes the coating powder out of the feed chamber through the powder outlet valve.
A different type of pump, which likewise relates to the invention, is an injector or injector pump respectively, by means of which coating powder is sucked into a conveying flow of air and then mixed with the conveying flow of air fed through a dispensing line (powder supply line) to a powder-spraying device. Such a powder feed can also be referred to as dilute phase powder conveyance.
This type of powder-dispensing device for dilute phase conveying of powder is known for example from printed publication EP 0 606 577 B1 or U.S. Pat. No. 4,284,032.
Specifically, the U.S. Pat. No. 4,357,900 printed publication discloses a powder spray coating device with which objects to be coated are transported through a chamber and are coated in the chamber by automatic spray devices subject to sensors, whereby a sensor serves to notify a control device when an object to be coated has been transported into the chamber so that the relevant powder spray device will then be activated.
In contrast to the dense phase conveying of powder, other conditions apply to dilute phase powder conveyance since it makes use of an injector as the powder pump to convey the coating powder. A flow of compressed conveying air creates a negative pressure in the injector. The negative pressure sucks coating powder into the flow of compressed conveying air. The mixture of compressed conveying air flow and powder then flows from the injector toward, for example, a powder-spraying device attached to the powder-dispensing device. The amount of powder delivered by the injector per unit of time depends on the volume of compressed conveying air flowing through the injector per unit of time.